Red, green, and blue emissions with high efficiency and high color purity are essential for achieving full color displays. Relative to red and green emissions, blue-emitting materials have poor color purity and efficiency. Introducing microcavity structure into organic light emitting device (OLED ) can improve the color purity and properties of the device. The microcavity structure of blue light OLED (MOLED) was optimized. MOLED was fabricated and studied with a structure of Glass/DBR/ITO/NPB/ DPVBi/Alq3/LiF/Al. Comparing the optimized microcavity (MOLED) to the noncavity devices (OLED) under the same current density, the electroluminescence (EL) peak intensity of the MOLED increased 5.4 times, the spectrum integral intensity increased 33%, while the full width at half maximum (FWHM ) decreased 4.4 times (only 16 nm), the maximum luminance of 8439 cd/m2, maximum luminous efficiency of 2.4 cd/A, and peak at 472 nm and the CIE coordinates (X=0.14, Y=0.10) were obtained in MOLED. It shows that MOLED can improve not only the color purity, but also the properties of the device. The microcavity effects such as spectra narrowing and intensity enhancement of spontaneous emission have been observed for MOLED.